Thymus-leukemia (TL) antigens will be studied on mouse thymocytes and leukemia cells, using three immunochemical techniques that distinguish between TL molecules in the plasma membrane that: (1) exhibit TL and antigens at the surface of the cell; (2) have TL antigens that are not so exposed; (3) TL molecules within the cell. These techniques have already indicated, in our hands, that normal thymocytes may differ from leukemic cells in regard to exposure of TL antigens, and that certain TL antigens may be present in the plasma membrane of thymocytes that are not detected by cytotoxicity assays. We propose to extend these initial observations to answer the question of whether all TL antigens of a given haplotype are expressed on the same or different molecules, to enquire further into the mechanism whereby TL minus cells acquire TL antigens as a result of leukemogenesis, and to study the process whereby TL plus cells undergo antigenic modulation in the presence of antibodies.